1. Technical Field
The present invention relates to a travel control unit of a working vehicle having a hydraulic continuously variable transmission including a hydraulic pump which is driven by an engine and a hydraulic motor which is fluidly connected to the hydraulic pump, and a controller.
2. Related Art
In the related art, a structure is known in which an engine and a hydraulic static continuously variable transmission (HST) are equipped in a working vehicle such as a farm tractor, a lawnmower, a wheel loader, or the like, wherein the hydraulic static continuously variable transmission includes a hydraulic pump and a hydraulic motor fluidly connected to the hydraulic pump. In this structure, the hydraulic pump is driven by an engine, and motive power of a rotational shaft of the hydraulic motor is speed-varied by a gear apparatus or the like and transmitted to a wheel of the vehicle. In addition, the hydraulic pump is of a variable displacement swash plate type in which an angle of the swash plate is changed to increase an amount of discharge of the hydraulic pump, to enable increase of a rotational velocity of the rotational shaft of a fixed displacement hydraulic motor.
JP 2001-71769 A discloses a hydraulic continuously variable transmission integrally constructed by a hydraulic pump and a hydraulic motor, wherein the capacities of the hydraulic pump and the hydraulic motor can be continuously adjusted. With a servo mechanism provided on a side of each of the hydraulic pump and the hydraulic motor, the swash plate angle of the movable swash plate of each of the hydraulic pump and the hydraulic motor is set to be rotatable. A rotation of a speed-varied operation arm of the servo mechanism is realized by manually operating through a link mechanism, an operation lever, or the like, or using a hydraulic piston.
JP 2006-64011 A discloses a hydraulic continuously variable transmission including a hydraulic pump and a hydraulic motor, wherein a variable displacement, movable swashplate type structure is employed for one or both of the hydraulic pump and the hydraulic motor. In this apparatus, a load control mechanism is provided which forms a hydraulic servo mechanism which controls the swash plate angle of the movable swash plate, which has an actuator for moving a pin for inclining the movable swash plate to a side of deceleration, and which guides pressure fluid of the main fluid path of a closed circuit which connects the hydraulic pump and the hydraulic motor to the actuator. The hydraulic motor is maintained at a maximum swash plate angle when stopped, and as the vehicle travels, the swash plate angle is reduced. In addition, a control is applied to reduce the swash plate angle by the load control mechanism.
During acceleration of a working vehicle equipped with the above-described hydraulic continuously variable transmission, when an acceleration pedal which is an acceleration operator is operated, the inclination angle of the movable swash plate of the hydraulic pump is increased with the operation of the acceleration pedal, and the rotational velocity of the hydraulic motor is increased. Because of this, the rotational velocity of the wheel linked to the rotational shaft of the hydraulic motor via a gear mechanism or the like in a manner to allow transmission of the motive power is increased, and the vehicle is accelerated. On the other hand, during the deceleration of the vehicle, when the acceleration pedal is not being operated, that is, the amount of depression of the accelerator pedal becomes 0 and the acceleration pedal is displaced to the side of low velocity, because the displacement of the hydraulic motor is fixed, the supply of the hydraulic oil from the hydraulic pump to the hydraulic motor is rapidly suspended, and the supply of the hydraulic oil from the hydraulic motor to the hydraulic pump is also rapidly suspended. Because of this, there will be no place for the hydraulic oil of the hydraulic motor to move to, causing a rapid deceleration of the hydraulic motor, and consequently, a rapid deceleration of the working vehicle. Therefore, it is desired to make the change of the velocity during deceleration of the working vehicle gradual, to reduce the change in the behavior of the vehicle, and to lessen a feeling of shock experienced by the driver during deceleration. In the vehicles having the hydraulic continuously variable transmission disclosed in JP 2001-71769 A and JP 2006-64011 A, there is still room for improvement from the viewpoint of lessening rapid deceleration of the vehicle when the acceleration operator is displaced on the side of low velocity.
When the rotational velocity of the wheel linked to the rotational shaft of the hydraulic motor through a gear mechanism or the like in a manner to allow transmission of motive power is increased as described above and the vehicle is accelerated, some users may wish to prevent the feeling of shock due to rapid acceleration regardless of the operation of the acceleration pedal when the vehicle starts to travel. In addition, the user may wish to change the travel starting performance between a work state where a working implement is used and a normal travel state where the working implement is not used. For example, when the working vehicle is a farm tractor, if the vehicle rapidly starts to travel while the working implement such as a tiller is working on the ground, an unfavorable state of the ground, such as a rough field, may result, and the user may wish to prevent such an unfavorable state. JP 2001-71769 A and JP 2006-64011 A do not disclose a means to facilitate provision of a travel starting performance according to the user's desires. For example, although JP 2006-64011 A discloses that the hydraulic motor of the HST is maintained at the maximum swash plate angle when stopped and the swash plate angle is reduced as the vehicle travels, the document does not disclose under what conditions such an operation is employed.